A dryer section of a paper machine receives a paper web to be dried, from the forming end and pressing section of the paper machine. The web is conventionally dried on a plurality of dryer rolls heated by steam or gas so as to reduce the moisture content of the web to an acceptable percentage. The moving paper web travels over and with the drying cylinders which, conventionally, have been arranged in two tiers with one side of the web contacting one cylinder and then the other side of the web contacting the next cylinder.
In recent times, drying sections of paper machines have the drying cylinders grouped entirely or largely in single tiered sections with all or most of the drying being done or one side of the web only. This can cause uneven or one sided drying of the web which can lead to a bowing or curl of the paper web.
As an example of conventional practice, the drying of paper and board is generally carried out on a multitude of steam heated cylinders. In this typical drying process, the web enters the drying section at 40 to 50% solids and leaves at 90 to 95% solids. Historically, the drying cylinders have been arranged in two tiers with the wet web travelling over first one cylinder, then the next, with one side of the paper heated over one drum and the opposite side on the next. The process continues until the paper exits the dryers at the desired solids content.
As the paper travels over the cylinders, it is held onto the cylinder by means of a dryer fabric the purpose of which is to guide the paper over the dryer and hold it in intimate contact with the dryer surface.
The dryers are normally divided into groups of six to ten cylinders, each group having its own drive system. Likewise the dryers are grouped to provide sectional steam control to allow the drying temperature to be varied through the complete dryer section. Each drive group has its own dryer fabrics, one for the top dryers and one for the bottom dryers. In a two tier dryer, the web is held against the dryer only for the duration or wrap of the fabric in the dryer, approximately 180.degree.. As the web passes from the one dryer to the next, it travels unsupported through the air (the draw) for a distance of several feet, until it reaches the next dryer and is then restrained by the fabric. While travelling through the draw, the web is susceptible to air movements which may cause the sheet to vibrate or flutter. If the aerodynamic forces acting on the web are high enough, the flutter may be sufficiently strong to cause the web to break which results in lost production while the broken paper (the broke) is removed and the web rethreaded through the machine.
Various solutions have been proposed to reduce sheet flutter in the dryer section and they have largely dealt with ways to reduce the length of the draw or unsupported length the paper must travel.
One solution is to use drying cylinders arranged in a single tier, with the conventional felt turn roll replaced with a larger vacuum roll. In this way the web travels restrained over one dryer then immediately over a turning roll which exerts a restraining force on the web, owing to the vacuum pressure acting on the back side of the web, then onto the next dryer and so on. In this way the unrestrained travel or draw is eliminated, the web stabilized and the detrimental aerodynamic effects minimized. The paper web can, in theory, be restrained through the duration of the drying process. Another advantage of this arrangement is that if the paper breaks in the drying section, the broke will automatically fall into the basement away from the machine, simplifying rethreading.
One drawback to this dryer arrangement is that it results in dryer sections that are very long compared to two tier dryer sections.
Another drawback to this solution arises from the one sidedness of the drying and is due largely to the formation of the paper web. The web is a mat of cellulose fibers which are more or less randomly oriented in the plane of the web, their orientation being dependent on the web formation. Likewise the paper web can be made up of two or more layers, each being comprised of different types of fiber, having different lengths and orientations.
As paper is dried, it tends to shrink. With the web being maintained in tension in the direction of the web travel, the shrinkage occurs in the width of the web, the majority of the shrinkage occurring as the web is dried from 60% solids to completion. The amount of cross machine shrinkage is proportional to the drying energy applied to the sheet.
In the case of a single tier drying section all of the drying energy is applied on one side of the sheet and as a result one side of the sheet tends to shrink more than the other. This non uniformity of shrinkage through the thickness of the web results in the web becoming bowed, a phenomena known as "curl". Curl is an undesirable condition as it leads to difficulties in the conversion of the web into a finished product, the "curled" web being more difficult to process than a flat web.
With two tiered dryers each side of the paper is dried more or less equally, however some degree of curl correction is often provided by having separate steam supply systems for the top and bottom dryers in one or more cylinder groups, curl correction being done by varying the shell temperature between the top and bottom dryers.
One method of avoiding curl is to dry on both sides which in a single tiered dryer section, requires that the dried side be reversed, at some point in the dryer section. Thus there is a series of dryer groups with the web dried on one side followed by another series of dryer groups with the web dried on the other side. However, this has some drawbacks from an operating stand point in that if the web should break in the section that has the felts on the bottom, the resulting broke must first be removed from the inside of the drying section before the sheet can be rethreaded, resulting in lost production time.
Another solution is to use dryers groups arranged in a single tier with the last section of dryers being a conventional two tiered dryer group. Although this entails the sheet travelling unsupported between the upper and lower tiers in the last section, the sheet, being largely dried at this location is sufficiently strong to resist the aerodynamic forces.
Still other solutions use a total single tier dryer section with curl correction taking place after the dryers by means of using steam shower to rehumidify the sheet and thus release some of the stresses in the web caused by the non-uniform drying.
This solution may be less than desirable because in order to have a dry web at the end of the process, the web must first be over dried in the dryer section, prior to rehumidification, which may result in a loss of production capacity.
The need for a solution to this curl problem has long been required. One proposed solution is found in a paper entitled "Defects Caused In Drying Paper--Notably Cockling And Curl" by G. H. Nuttall published in a book entitled "Drying Of Paper And Paperboard" 1972, Lockwood Publishing Co. Inc. This paper discloses several key causes of curl in paper web and it suggests possible solutions.
U.S. Pat. No. 5,600,898, Feb. 11, 1997, Deshpande et al, discloses a paper dryer for a single tier of top-felted dryer rolls which, in combination with air caps, simultaneously dry both sides of a web. The air caps are in the form of hoods which overlie the upper portions of the dryer rolls and blow high velocity hot air through the felts to dry the upper surface of the web, preferably at the same rate as the roll side of the paper is dried by steam heat transmitted to the surfaces of the dryer rolls.
U.S. Pat. No. 5,542,193, Beloit deals with a dryer group for controlling curl in the web. This is carried out by two separate groups of dryers. The patent discloses the use of a main line of single tier dryers to dry the first side of the web and then there is a further drying section which includes an upper tier of dryers and a lower tier of dryers to dry the second side of the web. A plurality of vacuum transfer rolls each of which is connected to a source of partial vacuum so that during the movement of the web around each of the vacuum transfer rolls the web is held against cross-machine and machine directional shrinkage. This patent also refers to the inclusion of control means to control the steam pressure within each dryer of the upper and lower tier thereof so that any tendency of the web to curl due to cross-machine directional shrinkage of the web is compensated for by the application of differential steam pressure.
Canadian Published application 2,136,901, Valmet, suggests several ways to dry the side of a web opposite to that which is dried on the face of the dryer drums. In one example the use of an infrared device is disclosed and the possible use of microwave or RF radiators are discussed. It is also suggested that it is possible to use devices for the blowing of drying air, such as a blow device, by means of which air jets are applied to the upper surface of the web.
Other patents of interest are as follows:
U.S. Pat. No. 4,523,390, Jun. 18, 1985 which discloses a peripheral exhaust system for a high velocity dryer having dryer hood sections.
U.S. Pat. No. 4,096,643, Jun. 27, 1978, Dominion Engineering Works, discloses a paper web drying system having a series of adjacent compartments extending transversely across the web guide with means to provide moisture profiling by varying the quantity, velocity and temperature of the drying medium.
U.S. Pat. No. 3,570,137 shows an apparatus for the continuous treatment of a web which encloses one or more web cylinders.
U.S. Pat. No. 3,183,607, May 18, 1985 discloses a drying hood having a movable plenum construction.
Other U.S. Patents which may be of interest to the reader are U.S. Pat. Nos. 3,208,158; 3,541,697; 3,377,056; 3,167,408; and 3,163,503. These disclose dryer hoods having plural headers for applying drying medium to their associated webs.